Poor quality frames

I bought a Stepper frame from Mellins in Bloemfontein at Preller Walk. It claims to be made from aluminium and that it is light, durable, corrosion resistant and have 3D shape molding. As I have a relative that is a optometrist at another major franchise, I only bought the frame at Mellins and took it to the family member to provide me with lenses and fit it to the frame. Whilst putting in the lenses in the frame no problems occurred, but after a few minutes the frame just cracked. I am very disappointed as I paid R1 900 for this frame and now Mellins claims it was the fault of the other franchise for not putting the lenses in correctly. This is quite ironic if you look at the claims that the frame is durable and 3D shape molding etc. Now they won't replace the frame for me and I lost R1 900. I feel this is unacceptable as I am still a paying customer and the frame has a one (1) year guarantee.